Read an FVCOM mesh on its own, with no data on it

accessFVCOM() returns values at points — nodes for scalars, element centroids for velocities. That is what matchData() needs, but it is not the mesh: it is a scatter of the mesh's points, and drawing it shows dots where the grid is triangles. This returns the triangles themselves, so the grid can actually be plotted.

Usage

fvcom_mesh(
  archive = "GOM3",
  bounding_box = NULL,
  what = c("polygons", "nodes", "elements"),
  date = NULL
)

Arguments

archive: which archive's mesh: a name from fvcom_archives(), or a spec from fvcom_archive()
bounding_box: optional named list with xmin, xmax, ymin, ymax, or an sf/sfc object. NULL returns the whole mesh, which for GOM3 is 90,415 triangles.
what: "polygons", "nodes", or "elements"
date: the month whose mesh to read, for an archive that remeshes. Ignored for a single-mesh archive.

Value

an sf object: POLYGON for "polygons", POINT otherwise, in EPSG:4326

Why the points are not enough

An FVCOM mesh is a triangulation, and which nodes form each triangle lives in a connectivity array (nv) that a point fetch never reads. Without it there is no way to draw a cell boundary, shade a cell by its value, or show how the resolution changes across a shelf — all of which are the usual reasons for looking at an unstructured model in the first place.

The result carries no time dimension and no covariates. It is the grid.

What comes back

"polygons" (the default) — one POLYGON per element, with the element index and DEPTH, the mean of its three nodes' bathymetry. This is what to plot.
"nodes" — the mesh nodes as POINTs, with DEPTH. Scalars such as SST and BOTS live here.
"elements" — the element centroids as POINTs. Velocities and stresses live here.

Bounding box

A triangle is kept when any of its three vertices falls inside the box, so the returned mesh covers the box rather than stopping short of it. The edge is therefore ragged, and a triangle may extend a little beyond what was asked for. Clipping to the box exactly would cut triangles into shapes the model does not have.

Meshes that move

GOM3 has one mesh for its whole record. GOM7 does not — it is operational output and remeshes between files, carrying 198,594 nodes in January 2025 and 207,081 from March. For an archive like that the mesh belongs to a particular month, so date says which; it defaults to the start of the record.

Examples

if (FALSE) { # \dontrun{
bb <- list(xmin = -70, xmax = -66, ymin = 41, ymax = 44)

mesh <- fvcom_mesh(bounding_box = bb)
plot(sf::st_geometry(mesh))                 # the grid itself
plot(mesh["DEPTH"], border = NA)            # shaded by bathymetry

# Shade the triangles by a fetched value. Join spatially rather than by
# position: accessFVCOM() returns only the points inside the box, so its row
# order does not correspond to the mesh's element numbering.
currents <- accessFVCOM(vars = "UBAR", years = 2010, months = 6,
                        bounding_box = bb)
shaded <- sf::st_join(mesh, currents["UBAR"], join = sf::st_contains)
plot(shaded["UBAR"], border = NA)
} # }